One aspect of the question that @Argenti didn't cover is Boost. Boost increases the spacing at the front fork (from 100mm to 110mm) and at the rear dropouts (from 142mm to 148mm). This is a mountain bike design standard, which increases wheel strength and allows for bigger tires. It does also move the chainline outward (i.e. away from the bottom bracket). Some discussion of the technical aspects is here.
Admittedly, I primarily ride road, and I am less familiar with the standard. However, it is commonplace on new mountain bikes. I suspect most of the cranks you encounter will be Boost compatible. Several people on this thread on the Mountain Bike Review forum contend that you can run a non-Boost crank on a Boost frame, although there might be some compromises. In any case, you do need to search for DUB Boost cranks, DUB being the spindle diameter (a SRAM standard, with a 28.99mm spindle, the rationale for which is a long story).
The question also asked the meaning of threaded. You do not need to change your bottom bracket if you buy a Boost DUB crank. However, you did ask. Threaded means, in this case, that the BB shell has threads in it. In Specialized's case, they correspond to the BSA thread standard - alternative threaded standards include the T47 oversized threaded standard, Italian threading, and some other niche ones like French that you'd only see on vintage bikes. Italian threading only exists on a very small number of Italian road bikes. I'm not certain that it's ever been offered on MTBs. It persists because some Italian manufacturers make very nice bikes, and they're Italian. An alternative to threaded BBs is press fit BBs (i.e. the cups are pressed into an unthreaded shell), but the shell has to be designed for press fit, and much ink has been spilled on the disadvantages of press fit. Anyway, if you acquired a crank with a different spindle type, e.g. a BB30 or 30mm spindle, or a Shimano 24mm spindle, you'd just change the bottom bracket to something with BSA threads that accepts that spindle type (but this could render your experiment uneconomical). Basically, stick to DUB cranks.
A side note on crank length itself. Humans are adaptable. There is some experimental evidence that our power production is pretty insensitive to crank length. With longer cranks, you may think you have more torque, but your cadence is lower. The reverse is true with shorter cranks. In road and triathlon, shorter cranks may be more conducive to aero positions because they open up your hip angle. However, I suspect most riders don't need to immediately go and change crank lengths. You could put the question to a bike fitter, which is always a worthwhile investment in making you more comfortable and more powerful on the bike. Experimenting with crank length can be expensive, after all.
